Colourimetric Fundamentals

Light + Object + Observer

The Colorimetry is defined as the measurement of colour. The measurement of colour allows coloured objects to be described, ordered, and compared. These operations must be accomplished in a logical and repeatable manner, in order to allow successful colour communications. And, successful colour communications are essential if satisfactory industrial colour control is to be accomplished.

Colour is an aspect of visual perception that is not easy to define, and certainly not easy to measure. It is a sensation whereby a human observer can distinguish differences between two fields of view, where such differences are caused by spectral composition differences in the observed radiant energies. From this it can be concluded that colour is:

  • A sensation, dependent upon the observer.
  • Only of interest if the observer can distinguish differences in sensations.
  • Caused by spectral (wavelength-by-wavelength) energy compositions (distributions).
  • The spectral energy compositions that are sensed by the eye/brain system of the human observer, result from both: Sources of light, and objects that modify light.

Although various systems have been developed for the measurement and ordering of colour, the most important system, by far, is the CIE system. First published in 1931, this colourimetric system is based on the principle that the colour of an object is a combination of light, object, and observer properties.  The CIE (Commission Internationale de l'Eclairage) is an international organization concerned with light and colour, that continues to further methods and standards concerning these subjects.

 

Elements Which Cause the Colour Stimulus

The CIE system is based on the premise that the stimulus for colour is provided by the proper combination of a source of light, an object, and an observer. The sensation of an object's colour is produced by the combination of:

  • A light source - illuminating an object.
  • An object - reflecting or transmitting light to an observer.
  • An observer - sensing the reflected light.
  • The combination of these three is considered on a spectral (wavelength-by-wavelength) basis.

 

Light Source

Electromagnetic energy exists as waves, which can be described by their wavelengths or frequencies. The wavelengths of these waves are distances, with 1 nanometer (nm) equal to 1e-9 meters. Humans can "see" electromagnetic energy over a range of wavelengths from about 400nm to 700nm. This part of the electromagnetic spectrum is called the visible (or colour) spectrum.

Light sources can be described by their relative energy outputs, wavelength-by-wavelength. These outputs are called relative spectral energy (or power) distributions. The colour producing effects of light sources result from the relative amount of energy available, not the absolute amount of the energy.

Light sources are also sometimes described by their correlated colour temperatures. The correlated colour temperature of a source is the temperature of a black body radiator that is most similar to the source. A blackbody radiator is an ideal surface that absorbs all energy incident upon it, and re-emits all this energy. The spectral output distribution of an incandescent (tungsten) lamp approximates a blackbody at the same temperature. Correlated colour temperature is typically presented using the absolute centigrade scale, degrees Kelvin (°K).

The CIE has published spectral output data for various illuminants, in order to facilitate and standardize colourimetric computations. These illuminants include:

  • D65 - daylight, colour temperature 6500° K.
  • A - tungsten, colour temperature 2856° K.
  • F2 - fluorescent, cool white.
  • F11 - fluorescent, narrow band cool white.

CIE Illuminant spectral output data is used in the process of calculating the colour of illuminated objects.

 

Optical Characteristics of Coloured Objects 

The spectral distribution of light reflected from an object depends upon:

  • The light illuminating the object; and
  • How the object modifies the incident light.
  • The spectral energy compositions that are sensed by the eye/brain system of the human observer, result from both: Sources of light, and objects that modify light.
  • For opaque objects, reflectance is determined by the following optical characteristics:
  • Surface reflection - diffuse (rough surface), or directional (smooth surface) reflection
  • Absorption - light enters the object and does not emerge (on a wavelength-by wavelength basis), as it is converted to heat.
  • Scattering - light enters the object and is deflected (on a wavelength-by-wavelength basis); and is then eventually absorbed, or exits the object.

The reflectance of an object is determined by a spectrophotometric measurement, with calibration relative to an ideal white, and perfect black. Spectral reflectance curves, graphical plots of the reflectance data, are often a useful way of presenting this information.

Reflectance data (of the object) is used in the process of calculating the colour of the object.

 

Observer

The human eye/brain system senses colour through three types of sensors (cones), located in the eye's retina. These cones are sensitive to light in three different wavelength bands, referred to as the L, M, and S bands. Processing of the cone signals, by the brain, eventually yields output sensations interpreted as red, green, and blue (and/or combinations and differences of these primary colours).

There are two CIE standard observers that can be used when computing CIE tristimulus values. They are as follows:

  • 2° Observer (CIE 1931) - for small objects.
  • 10° Observer (CIE 1964) - for large objects

The colour matching functions of these observers, with tabulated data wavelength-by-wavelength, are utilized in the tristimulus calculations.

 

Tristimulus Value

CIE tristimulus values X, Y, and Z, are coordinates of colour sensation, and form the foundation of the CIE colour space.

 

Acknowledgements:  Information offered in this datasheet has been obtained from Datacolor.com, Colour Learning: From The Experts.

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